Coagulation is a physiological pathway involved in maintaining normal blood hemostasis in mammals. Under conditions in which a vascular injury occurs, the coagulation pathway is stimulated to form a blood clot to prevent the loss of blood. Immediately after the vascular injury occurs, blood platelets begin to aggregate at the site of injury forming a physical plug to stop the leakage. In addition, the injured vessel undergoes vasoconstriction to reduce the blood flow to the area and fibrin begins to aggregate forming an insoluble network or clot, which covers the ruptured area.
When an imbalance in the coagulation pathway shifts towards excessive coagulation, the result is the development of thrombotic tendencies, which are often manifested as heart attacks, strokes, deep vein thrombosis, myocardial infarcts, unstable angina and acute coronary syndromes. Furthermore, an embolism can break off from a thrombus and result in a pulmonary embolism or cerebral vascular embolism including stroke or transient ischemia attack. Current therapies for treating disorders associated with imbalances in the coagulation pathway involve many risks and must be carefully controlled.
Heparin and low molecular weight heparins (LMWHs), complex, sulfated polysaccharides isolated from endogenous sources, are potent modulators of hemostasis. Heparin, a highly sulfated heparin-like glycosaminoglycan (HLGAG) produced by mast cells, is a widely used clinical anticoagulant, and is one of the first biopolymeric drugs and one of the few carbohydrate drugs. Heparin and molecules derived from it are potent anticoagulants that are used in a variety of clinical situations, especially for thromboembolic disorders including the prophylaxis and treatment of deep venous thrombosis and pulmonary embolism, arterial thromboses, and acute coronary syndromes like myocardial infarction and unstable angina. Heparin and LMWHs interact with multiple components of the coagulation cascade to inhibit the clotting process. Heparin primarily elicits its effect through two mechanisms, both of which involve binding of antithrombin III (AT-III) to a specific pentasaccharide sequence, HNAc/S,6SGHNS,3S,6SI2SHNS,6S contained within the polymer. First, AT-III binding to the pentasaccharide induces a conformational change in the protein that mediates its inhibition of factor Xa. Second, thrombin (factor IIa) also binds to heparin at a site proximate to the pentasaccharide/AT-III binding site. Formation of a ternary complex between AT-III, thrombin and heparin results in inactivation of thrombin. Unlike its anti-Xa activity that requires only the AT-III pentasaccharide-binding site, heparin's anti-III activity is size-dependent, requiring 1-13 saccharide units in addition to the pentasaccharide unit responsible for anti-Xa activity for the efficient formation of an AT-III, thrombin, and heparin ternary complex. Heparin also mediates the release of tissue factor pathway inhibitor (TFPI) from endothelial cells. TFPI, a heparin cofactor, is a serine protease that directly binds to and inhibits factor X. TFPI is a potent anti-thrombotic, particularly when co-administered with heparin.
In addition to heparin's anticoagulant properties, its complexity and wide distribution in mammals have lead to the suggestion that it may also be involved in a wide range of additional biological activities. Heparin-like glycosaminoglycans, present both at the cell surface and in the extracellular matrix, are a group of complex polysaccharides that are variable in length, consisting of a disaccharide repeat unit composed of glucosamine and an uronic acid (either iduronic or glucuronic acid). The high degree of complexity for HLGAGs arises not only from their polydispersity and the possibility of two different uronic acid components, but also from differential modification at four positions of the disaccharide unit. Three positions, viz., C2 of the uronic acid and the C3, C6 positions of the glucosamine can be O-sulfated. In addition, C2 of the glucosamine can be N-acetylated or N-sulfated. Together, these modifications could theoretically lead to 32 possible disaccharide units, making HLGAGs potentially more information dense than either DNA (4 bases) or proteins (20 amino acids). This enormity of possible structural variants allows HLGAGs to be involved in a large number of diverse biological processes, including angiogenesis (Sasisekharan, R., Moses, M. A., Nugent, M. A., Cooney, C. L. & Langer, R. (1994) Proc Natl Acad Sci USA 91, 1524-8, embryogenesis (Binari, R. C., Staveley, B. E., Johnson, W. A., Godavarti, R., Sasisekharan, R. & Manoukian, A. S. (1997) Development 124, 2623-32; Tsuda, M., Kamimura, K., Nakato, H., Archer, M., Staatz, W., Fox, B., Humphrey, M., Olson, S., Futch, T., Kaluza, V., Siegfried, E., Stam, L. & Selleck, S. B. (1999) Nature 400, 276-80.; and Lin, X., Buff, E. M., Perrimon, N. & Michelson, A. M. (1999) Development 126, 3715-23) and the formation of β-fibrils in Alzheimer's disease (McLaurin, J., Franklin, T., Zhang, X., Deng, J. & Fraser, P. E. (1999) Eur J Biochem 266, 1101-10. And Lindahl, B., Westling, C., Gimenez-Gallego, G., Lindahl, U. & Salmivirta, M. (1999) J Biol Chem 274, 30631-5).
Although heparin is highly efficacious in a variety of clinical situations and has the potential to be used in many others, the side effects associated with heparin therapy are many and varied. Anti-coagulation has been the primary clinical application for unfractionated heparin (UFH) for over 65 years. Due to its erratic pharmacokinetics following s.c. administration, UFH has been administered by intravenous injection instead. Additionally, the application of UFH as an anticoagulant has been hampered by the many side effects associated with non-specific plasma protein binding with UFH.
Side effects such as heparin-induced thrombocytopenia (HIT) are primarily associated with the long chain of UFH, which provides binding domains for various proteins. HIT is an immune-mediated thrombocytopenia which is the result of antibodies, usually IgG, directed against heparin-platelet factor 4 (PF4) complexes. Injected heparin binds with normally occurring low levels of PF4 in plasma to form a macromolecular complex that binds to the surface of platelets. In some patients, antibodies are produced against the heparin/PF4 complex. When present, these antibodies bind to the heparin/PF4 complex on the surface of platelets and crosslink Fc receptors on the platelet surface thereby causing platelet activation. Platelet activation releases procoagulants including additional PF4. Release of the latter in the presence of heparin further increases platelet activation. The activated platelets either join in forming a clot or are removed by the spleen. Platelet activation ceases when heparin is removed, however, the antibody usually remains detectable for four to six weeks.
Clinically, patients with HIT typically present with a decrease in platelet count, generally five to eleven days after initiated of heparin therapy. Platelet counts drop by up to 50%, to levels usually between 20 and 150 (×103/mm3). This thrombocytopenia is associated with thrombosis rather than purpura or bleeding; deep vein thromboses and pulmonary emboli are the most common complication. Arterial thrombosis occurs less often and usually involves large limb vessels, cerebral arteries, and visceral arteries. It has been estimated that 20% of patients receiving heparin therapy develop heparin induced platelet antibodies, 3% have a drop in platelet count, and 1% or less experience thrombotic complications. Other reported manifestations of heparin-induced thrombocytopenia include localized skin lesions with subcutaneous heparin administration, acute systemic reactions resembling febrile transfusion reactions, and transient global amnesia.
Other side effects include intracranial hemorrhage, bleeding, internal/external hemorrhage, hepatic enzyme (AST and ALT) level elevation, and derma lesion at the site of injection. This has led to the explosion in the generation and utilisation of low molecular weight heparin (LMWH) as an efficacious alternative to UFH. Although attention has been focused on LMWH as heparin substitutes due to their more predictable pharmacological action, reduced side effects, sustained antithrombotic activity, and better bioavailability, there is at present no means of correlating their activity with a particular structure or structural motif due to the structural heterogeneity of heparin and LMWH, as it has been technically unfeasible to determine their structures, and there has been no reliable and readily available means for monitoring LMWH levels in a subject. And since all of the commercially available LMWH preparations are not fully neutralized by protamine, an unexpected reaction could have extremely adverse effects; the anti-Xa activity of enoxaparin and other LMWH are neutralizable only to an extent of about 40% with ≦2 mg Protamine/100 IU anti-Xa LMWH. The anti-IIa activity is neutralizable only to an extent of about 60% with ≦2 mg Protamine/100 IU anti-Xa LMWH. (On the other hand, the anti-Xa and anti-IIa activity of UFH is neutralizable almost completely (>90%) with ≦2 mg Protamine sulfate/100 IU anti-Xa UFH.)
Pharmaceutical preparations of these polysaccharides, typically isolated from porcine intestinal mucosa, are heterogeneous in length and composition. As such, only a portion of a typical preparation possesses anticoagulant activity. At best, the majority of the polysaccharide chains in a pharmaceutical preparation of heparin or LMWH are inactive, at worst, these chains interact nonspecifically with plasma proteins to elicit the side effects associated with heparin therapy. Therefore, it is important to develop novel LMWHs that retain the anticoagulant activity and other desired activities of UFH but have reduced side effects. LMWHs, essentially due to their reduced chains sizes and dispersity, display markedly less non-specific plasma protein binding. However, all LMWHs that are currently clinically available also possess reduced anti-IIa activity as compared to UFH. Because of this decreased activity, a larger dose of LMWH is required (compared to UFH) in order to achieve a similar anti-coagulant activity, and the standard tests for UFH activity, activated partial thromboplastin time (aPTT) or thrombin clotting times (TCT), are not useful as they rely primarily on anti-IIa activity for a readout. The most widely used test for monitoring LMWH levels is an anti-Xa activity test, which depends on the subject having sufficient levels of antithrombin III (ATIII), which is not always the case. This test is quite costly (well over $100.00) and is not routine or readily available, as samples generally must be sent to an outside lab for analysis. Consequently, the use of LMWHs so far has been largely limited to the prevention of thrombosis and not to their treatment, and the population of patients to whom it can be administered has been limited, excluding, among others, pediatric patients, patients with abnormal renal function as measured by RFI, urea, creatinine, phosphorus, glomerular filtration rate (GFR), or BUN (Blood Urea Nitrogen level) in blood and urine and the interventional cardiology patient population. Improved monitoring methods are necessary to provide the advantages of LMWHs to a wider population of patients without increasing the risk of undesired effects. In addition, improved monitoring could allow for courses of therapy tailored to the patients condition throughout the course of their illness, for instance drug preparations given to the patient before a clot has been formed could differ from drug preparations given to the patient shortly after a clot has formed or a longer period of time after a clot has formed.
Although to a lesser degree than UFH, LMWHs are polydisperse and microheterogeneous, with undefined structure, and thus possess inherent variability. Current methods of LMWH preparation lack standardization and result in preparations that may vary substantially from batch to batch in composition and in efficacy.
In an attempt to characterize the molecular, structural, and activity variations of heparin, several techniques have been investigated for the analysis of heparin preparations. Gradient polyacrylamide gel electrophoresis (PAGE) and strong ion exchange HPLC (SAX) have been used for the qualitative and quantitative analysis of heparin preparations. Although the gradient PAGE method can be useful in determining molecular weight, it suffers from a lack of resolution, particularly the lack of resolution of different oligosaccharides having identical size. SAX-HPLC, which relies on detection by ultraviolet absorbance, is often insufficiently sensitive for detecting small amounts of structurally important heparin-derived oligosaccharides. As current technologies for analyzing heparins and other glycosaminoglycans are insufficient, it has been heretofore impossible to create LMWH preparations with any degree of batch-batch consistency, or to predict the potency of a given batch.